This invention relates to improved base coatings for use on paper to be vacuum metallized. It also relates to the resulting improved metallized paper, especially metallized bottle labels, prepared from the base coated paper and the process for their preparation.
As used herein, the term "paper" includes sheet-like masses made from fibrous cellulosic materials which may be derived from both natural sources as well as from synthetics such as polyamides, polyesters, and polyacrylic resins, and from mineral fibers such as asbestos and glass. In addition, papers made from combinations of cellulosic and synthetic materials are applicable herein. Paperboard is also included within the broad term "paper".
Vacuum evaporation or metallization is a process in which an "ultra-thin" layer of metal is deposited on a substrate such as plastic, glass, paper, and the like to achieve a metallic surface appearance. The bright, glossy appearance of metallized papers have lead to their use as decorative wraps and labels. Metallizing directly on paper, however, results in a totally unacceptable material, i.e., a paper with a dull surface. This is due to the rough texture of the paper surface, and even the most highly finished papers have microscopically rough surfaces. Since the deposited metal adheres directly to the surface of the substrate being metallized and also since the resulting metal coating thickness is about 250-300 Angstroms, it isn't surprising that direct metal deposition on paper yields an unacceptable product.
Hence, the paper's surface is typically coated with a base coating which fills minor surface imperfections and provides a smooth surface to receive the metal deposit. Among the properties which are important in base coatings are adhesion, water-resistance, and block-resistance (i.e., resistance to undesired adhesion between touching coated and noncoated surfaces, such as occurs under moderate pressure or sometimes under pressure and heat). While many of the base coatings currently used possess these properties and, in addition, provide the smooth, hard surface needed to give a highly reflective surface when vacuum metallized, the metallized papers may exhibit a high degree of sheet curl and may not be alkali-removable. Excessive sheet curl results in production losses due to feeding problems in high speed bottle labeling or other labeling operations. Poor alkali-removability is a major disadvantage for glass bottle labels since, in the recycling of the bottles, the entire label (metal and base coatings, as well as paper) must be removed during treatment with a dilute, hot alkali solution.
The present invention provides improved base coatings for use on paper to be vacuum metallized. It further provides improved metallized papers, especially aluminized bottle labels, which are characterized by their alkali removability, water-resistance, and block-resistance, as well as their smoothness and gloss.